There is known a high-voltage support insulator comprising an insulating ribbed core (in particular, made of porcelain) having sheds and, at its ends, metal flanges serving for fixation of the insulator to a high-voltage conductor and to a support structure (cf. High voltage techniques. Ed. D. V. Razevig, Moscow, “Energiya” Publishing House, 1976, p. 78).
A drawback of the prior art insulator consists in that, in an instance of a lightning overvoltage, a flashover of an air gap between metal flanges takes place,
and then under the influence of an operational frequency voltage that is applied to the high-voltage conductor the flashover transforms into a power arc of the operational frequency, which can damage the insulator.
There is further known a technical solution, aimed at protecting the above-described insulator from such a power arc. This solution consists of using so-called protective gaps (see “High voltage techniques”. Ed. D. V. Razevig, Moscow, “Energiya” Publishing House, 1976, p. 287) that are formed with the use of metal rods, that are electrically connected parallel to the insulator, with spark air gaps formed between the rods. The length of each of the spark gaps is less than a leakage path along the insulator surface, and less than a length of the flashover across air. Therefore, in an instance of the overvoltage, the flashover is formed not across the insulator, but across the air gap between the rods, so that the power arc of the operational frequency burns between the rods, and not across the insulator surface. A drawback of the insulator employing such protective gap consists in the fact that the flashover across the gap results in a short circuit of the connected power network, which necessitates the emergency shut-down of the high-voltage plant that contains the specified insulator.
There is also known an insulator string comprising two insulators which have rods fixed on their metal connecting terminals as protecting means against the arc formation. Such an insulating string, in contrast with the above-described insulator, additionally comprises a third intermediate rod electrode secured to a metal link in form of a length of chain between the insulators (see, for example, U.S. Pat. No. 4,665,460, H01T004/02, 1987). Thus, in such an insulating string, instead of a single spark air gap, two such gaps are formed. This feature made it possible to improve somewhat arc quenching ability of the insulator string equipped with the arc-protecting rods and to ensure the quenching of moderate follow currents (of the order of tens of amperes) in cases of single phase-to-ground short circuits. However, this device is unable to quench currents exceeding 100 A, which currents are typical for two- or three-phase-to-ground short circuits in lightning overvoltage cases.
From the technical aspects, the closest prior art for the invention is constituted by an insulator which has a cylindrical insulating core and spiral sheds. At the ends of the insulating core, first and second metal electrodes are fixed, while inside the insulating core a guiding electrode is located. This electrode has a metal protrusion located in the central part of the cylindrical body that emerges to the surface of the insulating core and functions as an intermediate electrode (cf. Russian patent No. 2107963, H01B17/14, 1998). In an instance of the lightning overvoltage in such an insulator, discharge develops across the surface of the cylindrical insulating core, along a spiral path from said first metal electrode through the intermediate electrode to said second metal electrode. Due to the increased length of the flashover path, a power arc is not formed by the operational frequency voltage, and therefore, the electric plant that contains the insulator continues functioning without shutting down. Thus, in addition to its primary function, such an insulator also provides lightning protection, i.e. functions as a lightning arrester.
However, effectiveness of the prior art insulator as the lightning arrester is limited for the reason that, in cases of substantial atmospheric pollution and/or moisture accumulation, as well as in cases of large overvoltages (exceeding 200 kV), the discharge does not develop along the long spiral path, but along the shortest trajectory, with a breakdown of air gaps between sheds. In such instances, the insulator loses its ability to function as the lightning arrester because, same as in a conventional insulator, the flashover in this insulator transforms into a power arc. In addition, the metal protrusion located in the central part of the insulating core decreases the leakage path and, therefore, decreases allowable voltage for such insulator. Thus, its effectiveness as an insulator is also limited.
There are also known various HEPLs employing combinations of high-voltage insulators (for securing conductors to supports, such as towers or poles) and lightning arresters for protecting such insulators (cf., for example, Russian patent No. 2248079, H02H9/06, 2005, assigned to the applicant of the present invention). In particular there are known the HEPLs comprising the lightning arresters which are configured as various impulse arresters and connected parallel to the insulators (see for example, U.S. Pat. No. 5,283,709, H02H001/00, 1994, and RU 2002126810, H02H9/06, 2004).
As for the closest prior art for the proposed technical solution, the HEPL that may be indicated is disclosed in Russian patent No. 2096882, H02G7/00, 1997 (assigned to the applicant of the present invention). The prior art HEPL comprises supports, insulators secured to the supports by means of metal fixing devices, at least one conductor operating under a high voltage, the conductor being connected to the insulator by means of coupling means, and means for protecting the insulators against lightning overvoltages, said means configured as impulse arresters.
If the impulse arresters are properly selected and connected, the prior art HEPL ensures a highly reliable lightning protection. However, a necessity to use a large number of the impulse arresters substantially increases the complexity of the HEPL, with a corresponding increase of manufacturing and assembling costs.